1. Field of the Invention
The invention relates to a signal processing circuit for an electrostatic capacitor type touch sensor.
2. Description of the Related Art
A touch sensor is conventionally known as an input device for various types of electronic device such as a cellular phone, a portable audio device, a portable game device, a television or a personal computer.
FIG. 10 shows a touch sensor including a touch panel 1, and m numbers of X lines XL1 to XLm are formed on the touch panel 1 in the X direction, and n numbers of Y lines YL1 to YLn are formed in the Y direction so as to cross the X lines XL1 to XLm. The X lines XL1 to XLm and the Y lines YL1 to YLn are insulated by an insulation layer and are capacitively coupled.
An X sensor IC 2 is an IC that detects a capacitance change occurring in the X lines XL1 to XLm by the touch of a human finger, and a Y sensor IC 3 is an IC that detects a capacitance change occurring in the Y lines XL1 to XLn by the touch of a human finger.
For example, when a human finger touches a point P1 that is the intersection of the X line XL1 and the Y line YL1 in FIG. 10, the X sensor IC 2 detects the capacitance change of the X line XL1 and outputs a detection signal, and the Y sensor IC 3 detects the capacitance change of the Y line YL1 and outputs a detection signal. The touch to the point P1 is thus detected. A relevant technique is described in Japanese Patent Application publication No. 2005-190950.
In the touch sensor described above, however, in a case of multiple touches such as when two points are touched at the same time, there arises a problem that the two points are not distinguished. For example, when the points P1 and P2 in FIG. 10 are touched at the same time, the X sensor IC 2 detects the capacitance changes of the X lines XL1 and XL2 and outputs detection signals for these, and the Y sensor IC 3 detects the capacitance changes of the Y lines YL1 and YL2 and outputs detection signals for these. Since the same detection signals are also outputted when the points P3 and P4 in FIG. 10 are touched at the same time, these are hardly distinguished.